The present invention relates to a method for making the drying of materials, particularly finely divided materials, more effective by subjecting the filter cake made of said materials to the influence of an acoustic field.
A restrictive factor in the filtering of finely divided materials, such as fine sulfide concentrates, titanium pigments, kaoline or calcite, is the high flow resistance of the filter cake as well as its large inner area, which binds large amounts of liquid to be filtered. Naturally the residual moisture also is affected by capillaries formed by tiny particles and their capillary forces. Thus the filtering capacity remains low, and the residual moisture in the filter cake remains high.
The residual moisture of the filter cake is bound to the cake material by different mechanisms. Part of the liquid to be filtered is adsorbed on the surface of the particles as a thin layer. The removal of this liquid is not possible without thermal energy. Part of the liquid is bound to the capillary networks formed by the particles. Depending on the surface chemistry of said particles and on the diameter of the capillary, with a given pressure difference, there is left a given amount of liquid. By increasing the pressure difference, liquid can be removed through smaller and smaller pores, which reduces the residual moisture. Moreover, part of the liquid is bound to extremely narrow capillaries and particle contact points, as well as to closed pores.
In the WO application 96/35,340, there is described a method for drying foodstuffs and other materials, said method applying a combination of hot air and ultrasonic oscillation. The products to be dried are transported through a closed chamber, so that inside the chamber, the products are subjected to the influence of hot air. In the chamber, ultrasonic waves are conducted to the products by means of flat ultrasonic oscillators, which are in direct contact with the products to be dried. The method described in said WO application 96/35340 is designed for materials, such as vegetables, that are dried as whole items. In that case the specific area to be dried is very small in comparison with finely divided materials.
The object of the present invention is to eliminate some of the drawbacks of the prior art and to achieve a new, more effective method for drying filtered materials, particularly finely divided materials with a particle size smaller than 100 micrometers, so that the filter cake made of said material is subjected to the influence of an acoustic field in order to reduce the residual moisture of the filter cake. The essential novel features of the invention are apparent from the appended claims.
In the method according to the invention, of the finely divided solids present in the slurry, there is first produced a filter cake onto the surface of the filter medium. According to the invention, the produced filter cake can be further dried by means of oscillation generated by an acoustic field, wherefore the filter cake and the oscillator are further arranged, with respect to each other, in a position where a mechanical contact is achieved between the filter cake and the oscillator used for effecting the drying process, or a structural element made to oscillate by means of said oscillator. The mechanical contact between the oscillator or the structural element made to oscillate by means of said oscillator and the filter cake creates an acoustic coupling. By virtue of said acoustic coupling, the residual moisture still left in said filter cake can be reduced, and thus the filtering result further improved, because the acoustic field created by said acoustic coupling causes oscillation in the filter cake, and consequently removes water, which is otherwise difficult to be removed.
In the method according to the invention, the surface of the oscillator or of the structural element made to oscillate by means of said oscillator that mechanically touches the filter cake is set, with respect to the filter cake, in a position where the filter cake, essentially throughout the whole surface opposite to the filter surface, is in mechanical contact with the surface of the oscillator or of the structural element made to oscillate. Advantageously the mechanical contact is achieved so that the filter cake is made to move past the permanently installed oscillator or the structural element made to oscillate by means of said oscillator, in which case the surface of the filter cake opposite to the filter medium makes, at least from time to time, a mechanical contact in order to create an acoustic field.
A mechanical contact, and hence an acoustic field, between the oscillator or the structural element made to oscillate and the filter cake can also be achieved so that the oscillator or the structural element made to oscillate is moved with respect to the filter cake. The moving of the oscillator or the structural element made to oscillate with respect to the filter cake is carried out for instance so that the oscillator or the structural element made to oscillate is transferred towards that surface of the filter cake that is opposite to the filter medium, and the motion of the oscillator or the structural element made to oscillate is stopped, when a mechanical contact is made. Now the acoustic field can be essentially simultaneously focused to substantially the whole filter cake. The moving of the oscillator or the structural element made to oscillate with respect to the filter cake can also be carried out so that the oscillator or the structural element made to oscillate is moved past that surface of the permanently installed filter cake that is opposite to the filter medium, so that there is achieved a mechanical contact between the filter surface and the oscillator or the structural element made to oscillate in order to create an acoustic field.
The oscillation frequency applied in the method according to the invention is advantageously within the range of 20-60 kHz. Lower frequencies can also be applied, but these lower frequencies may lead to environmental hazards because of the extremely high noise level caused by the low frequency. The amplitude used in connection with the oscillation frequency is inversely proportional to the oscillation frequency, so that with a higher frequency, a lower amplitude is applied.
Advantageously the oscillator employed in the method according to the invention is an ultrasonic oscillator, for instance. In order to generate oscillation, there can also be used a piezoelectric or a magnetostrictive material, either as the oscillator itself, or in the structural element connected to said oscillator.
The materials that are suitable to be used in the method according to the invention advantageously have a particle size within the range of 0.5-10 micrometers. Such materials are for example sulfidic concentrates of copper and zinc, titanium pigments, kaoline, calcite and organic compounds, such as starch. However, depending on the structure of the material, the particle size range advantageous for the application of the method can be remarkably larger. In that case the particle size range may be 0.01-100 micrometers.